Referring to FIG. 3, a diagram of a typical solar collecting system, solar thermal units 300, also known as solar thermal collectors, solar energy collectors, solar panels, or solar modules, convert solar radiation to thermal energy for a variety of applications 302 within residential or industrial structures. Typical applications include water heating 304, space heating 306, industrial process heating 308, solar cooling 309, and other applications 310. A variety of solar thermal collectors are commercially available, and deployment, operation, and maintenance of conventional solar thermal collectors is well known in the industry. For clarity in this document, the singular term application is often used but does not imply limiting to a single application, and one ordinarily skilled in the art will understand that that multiple applications are included. In the context of this document, references to the term solar collecting system generally refer to one or more solar thermal collectors, application components, and related support components.
Thermally insulating panels transmissive to solar radiation, while having low transmissivity to thermal infra-red radiation, have been disclosed in U.S. Pat. Nos. 4,480,632, 4,719,902, 4,815,442, 4,928,665, and 5,167,217 all to Klier and Novik. The thermally insulating panel, also called transparent insulation material or thermal diode, may be a honeycomb made of synthetic material or a honeycomb made of glass, which is transparent to solar infrared (IR) radiation and visible wavelength light, while being opaque to thermal IR back-radiation, as a result of the optical properties of the material and the geometry of the material and/for the panel. At the same time, the transparent insulation material is a thermal convection suppressor as a result of the geometry and physical characteristics of the material, and a thermal conduction suppressor as a result of the thermal properties of the material, including for example, thin walls of a honeycomb.
This imbalance of the transparency to incoming solar radiation versus the thermal IR back-radiation and the restricted energy losses due to low convection and conduction create a thermal diode and enable the capturing of heat and use of that captured heat for a variety of energy applications. The use of thermal insulation panels enables much greater energy conversion efficiencies over a much broader range of ambient temperatures and conditions, especially in colder climates as compared to systems that do not use a thermal insulation panel. In certain implementations, the solar absorption surface is coated with a spectrally selective layer that suppresses thermal re-emission in the thermal infrared spectrum, obviating the need for the transparent insulation to be substantially opaque in the thermal infrared spectrum.
A solar thermal collector with transparent insulation material is known as an insulated solar panel. In this case “insulated” refers to the transparent insulation material, as opposed to the conventional insulation typically used in the back and sides of a solar thermal collector. Insulated solar panels are available from TIGI of Neve Yarak, Israel. An insulated solar panel provides a solar thermal collector with much greater energy conversion efficiencies, as compared to conventional solar thermal collectors. This occurs particularly under conditions of substantial temperature differentials between the ambient temperature and the temperature of the circulating fluid (for example, heated water) inside the collector, for example in cold, high latitudes in winter. Referring to FIG. 8, a plot of collector efficiency (h) as a function (X) based on temperature (where X=ΔT/G where ΔT is the temperature difference between ambient and the average collector temperature and G is the global solar radiation), higher values of X indicate colder and less sunny conditions. As can be seen from FIG. 8, the efficiency of an insulated solar panel remains high as the environment gets colder and/or the amount of available solar radiation decreases, as compared to a conventional flat panel collector. When the efficiency of a typical conventional flat panel collector drops to about zero (for example, in the range of 0 to 10%), an insulated solar panel can still operate at an efficiency of about 40%. Greater efficiencies in insulated solar panels, while providing greater benefits than conventional solar panels, also create operational and maintenance challenges that must be addressed for successful operation.
There is therefore a need for innovative solutions to address the new challenges of operating insulated solar panels, in particular internal temperature limiting in the insulated solar panel to prevent degradation or catastrophic damage.